Recent advance in imaging techniques, e.g.functional MRI and PET, allows us to understand the activation of various brain areas.However, due to the spatial limitation of these approaches, it is difficult to observe more fine structures, e.g.neurons and synapses in vivo.Two photon excitation of fluorescent molecules enables us to observe the fine structures and neuronal activity in the various organs and tissues.Here, we introduce two evidences of dynamics of neuronal structures and glia in the brain of living mice.Real time imaging with two photon microscopy revealed that resting microglia, the primary immune effector cells in the brain, dynamically extend and retract their processes as if actively surveying the microenvironments.Microglia directly monitors the local synaptic states.